Investments - Commercial Mortgage Loan Participations (Details) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020 USD ($) loan	Jun. 30, 2019 USD ($)	Jun. 30, 2020 USD ($) loan	Mar. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)	Mar. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)
Accounts, Notes, Loans and Financing Receivable [Line Items]
Accounting Standards Update [Extensible List]	us-gaap:AccountingStandardsUpdate201613Member	us-gaap:AccountingStandardsUpdate201602Member	us-gaap:AccountingStandardsUpdate201613Member
Commercial mortgage participations
Accounts, Notes, Loans and Financing Receivable [Line Items]
Carrying value, gross	$ 168,258		$ 168,258		$ 137,692
Carrying value, gross, percent	102.00%		102.00%		100.00%
Allowance for credit losses	$ (3,838)	$ 0	$ (3,838)	$ (335)	$ 0	$ 0	$ 0
Allowance for credit losses, percent	(2.00%)		(2.00%)		0.00%
Carrying value, net of valuation allowance	$ 164,420		$ 164,420		$ 137,692
Carrying value, net, percent	100.00%		100.00%		100.00%
Number of loans | loan	27		27
South Atlantic | Commercial mortgage participations
Accounts, Notes, Loans and Financing Receivable [Line Items]
Carrying value, gross	$ 52,003		$ 52,003		$ 50,867
Carrying value, gross, percent	32.00%		32.00%		37.00%
Pacific | Commercial mortgage participations
Accounts, Notes, Loans and Financing Receivable [Line Items]
Carrying value, gross	$ 59,042		$ 59,042		$ 36,546
Carrying value, gross, percent	36.00%		36.00%		27.00%
Middle Atlantic | Commercial mortgage participations
Accounts, Notes, Loans and Financing Receivable [Line Items]
Carrying value, gross	$ 30,514		$ 30,514		$ 25,328
Carrying value, gross, percent	18.00%		18.00%		18.00%
East North Central | Commercial mortgage participations
Accounts, Notes, Loans and Financing Receivable [Line Items]
Carrying value, gross	$ 10,589		$ 10,589		$ 10,568
Carrying value, gross, percent	6.00%		6.00%		8.00%
West South Central | Commercial mortgage participations
Accounts, Notes, Loans and Financing Receivable [Line Items]
Carrying value, gross	$ 8,388		$ 8,388		$ 8,072
Carrying value, gross, percent	5.00%		5.00%		6.00%
East South Central | Commercial mortgage participations
Accounts, Notes, Loans and Financing Receivable [Line Items]
Carrying value, gross	$ 4,692		$ 4,692		$ 4,676
Carrying value, gross, percent	3.00%		3.00%		3.00%
West North Central | Commercial mortgage participations
Accounts, Notes, Loans and Financing Receivable [Line Items]
Carrying value, gross	$ 1,389		$ 1,389		$ 0
Carrying value, gross, percent	1.00%		1.00%		0.00%
New England | Commercial mortgage participations
Accounts, Notes, Loans and Financing Receivable [Line Items]
Carrying value, gross	$ 1,641		$ 1,641		$ 1,635
Carrying value, gross, percent	1.00%		1.00%		1.00%
Office | Commercial mortgage participations
Accounts, Notes, Loans and Financing Receivable [Line Items]
Carrying value, gross	$ 42,997		$ 42,997		$ 42,350
Carrying value, gross, percent	26.00%		26.00%		31.00%
Mixed use | Commercial mortgage participations
Accounts, Notes, Loans and Financing Receivable [Line Items]
Carrying value, gross	$ 44,892		$ 44,892		$ 27,501
Carrying value, gross, percent	27.00%		27.00%		20.00%
Hospitality | Commercial mortgage participations
Accounts, Notes, Loans and Financing Receivable [Line Items]
Carrying value, gross	$ 22,736		$ 22,736		$ 22,324
Carrying value, gross, percent	14.00%		14.00%		16.00%
Industrial | Commercial mortgage participations
Accounts, Notes, Loans and Financing Receivable [Line Items]
Carrying value, gross	$ 22,439		$ 22,439		$ 17,612
Carrying value, gross, percent	14.00%		14.00%		13.00%
Retail | Commercial mortgage participations
Accounts, Notes, Loans and Financing Receivable [Line Items]
Carrying value, gross	$ 18,899		$ 18,899		$ 17,318
Carrying value, gross, percent	11.00%		11.00%		12.00%
Multi-family | Commercial mortgage participations
Accounts, Notes, Loans and Financing Receivable [Line Items]
Carrying value, gross	$ 16,295		$ 16,295		$ 10,587
Carrying value, gross, percent	10.00%		10.00%		8.00%